LEDs are rapidly replacing incandescent bulbs, fluorescent bulbs, and other types of light sources due to their efficiency, small size, high reliability, and selectable color emission. A typical forward voltage drop for a high power LED is about 3-4 volts. The brightness of an LED is controlled by the current through the LED, which ranges from a few milliamps to an amp or more, depending on the type of LED. For this reason, LED drivers typically include some means to control the LED current.
In applications where high brightness is needed, multiple LEDs are used. It is common to connect LEDs in series, since the current through all the LEDs in series will be the same. However, driving many LEDs in series requires relatively high voltages, since the driving voltage must be greater than the combined voltage drops of the series LEDs. Additionally, when different types of LEDs are driven, such as different colors of LEDs, it may be desired to drive each type of LED with a different current. Therefore, in some applications, LEDs are connected in parallel, with the current through each parallel path being separately controlled. There may be only one LED in each parallel path, a few LEDs connected in series in each parallel path, or another configuration of LEDs in each parallel path. In some cases, one parallel path may drive a single red LED, another parallel path may drive two or more green LEDs connected in series, and another parallel path may drive one or more blue LEDs, in order to achieve a target overall brightness and color. Parallel LEDs may also be used for redundancy in case one LED fails and becomes an open circuit.
FIG. 1A is a typical prior art LED driver 10 that drives multiple LEDs 12 in parallel. Some parallel paths show multiple LEDs connected in series. Most components of the driver 10 are formed on an integrated circuit chip. The LEDs are typically connected to pins extending from the chip package.
A DC voltage regulator controller 14 up converts or down converts an input voltage (Vin), depending on the required voltage to drive the LEDs 12. Typically, the voltage regulator is a switching regulator operating at a high frequency, such as 100 KHz-5 MHz, to keep component sizes small. The controller 14 switches a switching transistor at a certain pulse-width modulation (PWM) duty cycle to maintain an output voltage (Vout) at the desired level. The switching transistor and an output circuit are represented by block 16. The output circuit comprises an inductor and diode (or synchronous rectifier) connected to the switching transistor to supply pulses of current to a smoothing capacitor 18. The topology of the output circuit determines whether the voltage regulator is a step up or step down regulator. Such regulators are well known and need not be described in detail.
The anodes of the “top” LEDs in each parallel path are connected to Vout, as shown in FIG. 1A, and the cathodes of the “bottom” LEDs in each path are connected to designated pins of the driver 10. The current through each path is individually set, such as by the user connecting a current set resistor to corresponding pins of the driver 10. Other types of current setting circuits may be used. A current setting circuit 20-22 for each parallel path is shown, where each circuit 20-22 may comprise a current set resistor or a current regulator. It is important to individually set the currents, since each parallel path may drive a different type (e.g., color) of LED that requires a different current to achieve the desired light output.
An LED's color is slightly dependent on the magnitude of the forward current. Once the current is fixed, it should be not changed in order to avoid color shift. Therefore, to adjust the perceived brightness of the LEDs 12, a PWM brightness control unit 24 outputs a PWM signal at a relatively low frequency (e.g., 100 Hz-1000 Hz) and effectively turns all the LEDs on and off at the LF PWM duty cycle. An externally generated dimming control signal sets the LF PWM duty cycle. For example, if the duty cycle were 50%, the average current would be half of the peak current when the LEDs are on. Thus, the perceived brightness of the LEDs would be about half the brightness of the LEDs when fully on.
FIG. 1B illustrates one type of brightness control unit 24. An N-Bit counter 26 cyclically counts, such as a repeating 5-bit count from 0-31, at a rate set by a clock. The binary count is applied to preset count detectors 28 and 30. Detector 28 detects a count that begins the high state of the PWM cycle, such as the count 00000. Upon count 00000 being detected, the detector 28 outputs a pulse. Detector 30 detects a count that begins the low state of the PWM cycle, such as 01010. Upon count 01010 being detected, the detector 30 outputs a pulse. The outputs of the detectors 28 and 30 are connected to the set and reset inputs, respectively, of an RS flip flop 32. The output of the flip flop 32 is the PWM brightness control signal, where the duty cycle in the example is approximately 30%.
The frequency of the LF PWM signal must be high enough to prevent noticeable flicker of the LEDs. There may be thousands of HF PWM pulses generated by the voltage regulator during each pulse (on-time) of the LF PWM signal. In this way, the current through the LEDs is either a “fixed” peak current or zero current. This keeps the emitted color of the LEDs constant, but enables the perceived brightness to be adjusted by the LF PWM duty cycle.
Since there are multiple parallel sets of LEDs, the combined currents of the paths can be high. For example, some LEDs may be driven at 1 Amp. When the brightness control PWM signal goes high, the voltage regulator must instantaneously supply driving current to all the LEDs at the same time. The large instantaneous change in the current causes a voltage ripple in the battery or other power supply. Greatly increasing the value of the smoothing capacitor 18 to smooth out the current drain on the power supply is not desirable, since such high value capacitors are relatively large and expensive. The ripple in the power supply due to the high currents being switched on and off causes noise that affects other circuits connected to the power supply. Since the PWM brightness control signal is at audio frequencies, the noise may even be audible.
What is needed is an LED driver that produces less ripple and noise compared to the conventional LED drivers.